For mobile communication transmissions of today, such as MIMO, it is often desirable to use dual polarized antennas. Generally, a dual polarized antenna comprises a first number of first antenna elements, having a first polarization, and a second number of second antenna elements having a second polarization. Normally, the first and second numbers are equal, and the first polarization and second polarization are mutually orthogonal, constituting a number of dual polarized antenna elements. The first antenna elements are connected to a first antenna beam port, and the second antenna elements are connected to a second antenna beam port. Depending on the number of antenna elements, corresponding distribution networks are used.
Often the first and the second polarizations are provided with dual orthogonal polarized antenna elements, where the first polarization is associated with the first antenna beam port and the second polarization is associated with the second antenna beam port.
The antenna radiation patterns of the antenna elements of each polarization may be tilted electrically by feeding each antenna element with a certain phase. Such an electrical tilt requires that at least two antenna elements are used for each polarization. The electrical tilt may be fixed or adjustable, and set by means of how the distribution network is designed. In some cases also a certain amplitude is applied to each antenna element for side-lobe control.
For each polarization of a dual polarized sector covering antenna having a number of antenna elements in a vertical column, there is normally a broad coverage in azimuth, perpendicular to the longitudinal extension of the antenna column, in a broad main antenna beam. In elevation, along the longitudinal extension of the antenna column, there is normally a relatively narrow coverage in a narrow antenna beam with adjacent side-lobes. In so-called null directions in elevation there is a very low antenna gain, between the side-lobes. In these directions, so-called nulls are present in the antenna radiation pattern. These nulls are present for both polarizations. Consequently, the path-gain is quite low in these null directions.
It is desirable to increase the path-gain in the null directions, with maintained orthogonality between the two radiation patterns comprised in the array antenna.
In WO 2006/065172 single polarized sub-array antennas, each sub-array antenna comprising a number of antenna elements having a certain polarization, are mounted in such a way that they constitute a total array antenna. It is suggested that a sub-array having a different polarization is mixed with the others in order to provide null-fill.
The document WO 2006/065172 only concerns a single polarized antenna, and the proposed solution for null-fill requires one additional sub-array.
There is thus a need for a dual polarized antenna with increased path-gain in the null directions, with maintained orthogonality between the polarizations.